Target PSStandard at startup to allow older PowerShell Core versions (#1132)

* Target PSStandard at startup to allow older PowerShell versions

* Mark private assets

Author: rjmholt

src/PowerShellEditorServices.Hosting/PowerShellEditorServices.Hosting.csproj

@@ -12,6 +12,7 @@
 
   <ItemGroup>
     <PackageReference Include="NETStandard.Library" Version="2.0.3" />
+    <PackageReference Include="PowerShellStandard.Library" Version="5.1.1" PrivateAssets="all" />
     <PackageReference Include="System.IO.Pipes.AccessControl" Version="4.5.1" />
     <PackageReference Include="System.Runtime.InteropServices.RuntimeInformation" Version="4.3.0" />
   </ItemGroup>
@@ -24,13 +25,7 @@
     </PackageReference>
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFramework)'=='netcoreapp2.1'">
-    <PackageReference Include="System.Management.Automation" Version="6.2.3" PrivateAssets="all" Private="false" />
-
-  </ItemGroup>
-
   <ItemGroup Condition="'$(TargetFramework)'=='net461'">
-    <PackageReference Include="Microsoft.PowerShell.5.ReferenceAssemblies" Version="1.1.0" PrivateAssets="all" Private="false" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
 
     <Compile Remove="Internal/PsesLoadContext.cs" />